EP0649181B1 - Antenna for portable radio apparatus, method for manufacturing the same and portable radio apparatus comprising the same - Google Patents

Abstract

The present invention relates to an antenna (1) of the type for a portable radio device, particularly comprising a helical antenna (2) coupled by its base to a transmitter/receiver, characterised in that the pitch of the helix (8) made of a conducting material constituting the helical antenna (2) is variable according to the height of the helix, and decreasing from the base (8B) of the helical antenna up to its peak (8A). <IMAGE>

Description

The present invention relates to an antenna of the type for portable radio device, and in particular for radiotelephone, as well as a method of manufacturing a such an antenna and that a portable radio device comprising such an antenna. This antenna is intended to transmit and receive radio signals.

• une antenne hélicoïdale quart d'onde disposée à la partie supérieure du boítier du radiotéléphone et alimentée par une ligne coaxiale couplée à l'émetteur/récepteur du dispositif radio, pour l'utilisation dans des conditions normales, l'hélice étant en général constituée d'un fil métallique bobiné autour d'un mandrin support en un matériau isolant,
• éventuellement, pour l'utilisation du dispositif radio dans des conditions fortement perturbées, un brin demi-onde extractible hors du boítier du dispositif radio, tel que, lorsqu'il se trouve en position rentrée à l'intérieur du boítier, il est pratiquement entièrement découplé de l'antenne hélicoïdale, et lorsqu'il se trouve en position déployée hors du boítier, il est couplé de manière capacitive à l'antenne hélicoïdale.

The antennas currently used in portable radio devices, and more particularly in radiotelephones, generally include:

• a quarter-wave helical antenna placed at the top of the radiotelephone case and supplied by a coaxial line coupled to the transmitter / receiver of the radio device, for use under normal conditions, the propeller generally being made up of: '' a metal wire wound around a support mandrel made of an insulating material,
• possibly, for the use of the radio device in highly disturbed conditions, a half-wave strand extractable from the housing of the radio device, such that, when it is in the retracted position inside the housing, it is practically entirely decoupled from the helical antenna, and when it is in the deployed position outside the housing, it is capacitively coupled to the helical antenna.

Such antennas are described for example in the patent application EP-0 367 609 and in the patent US-4,121,218.

The radio performance of such antennas, although that they are acceptable for their use are not optimal, especially in terms efficiency and bandwidth. This is due to the fact that their radiation impedance, characteristic of their radiant power and therefore their effectiveness in as long as antennas, is weak (in practice much less than 50Ω).

On the other hand, given the current size relatively small portable radio it is desirable, to occupy as little space as possible in the radiotelephone case, decrease as much as possible the size of the extractable strand, the latter being housed at the inside of the case when it is in position September. Indeed, the volume occupied inside the case by the extractable strand cannot be occupied by others elements necessary for the operation of the radiotelephone (transmitter / receiver, modulator / demodulator, encoder / decoder, smart card connector, etc.).

Now the extractable strands currently known are generally substantially cylindrical, so they occupy too much volume in the housing radiotelephone.

In addition, the performance in terms of gain and omnidirectionality of known antennas of the previous type using an extractable strand are deteriorated by the dependence between the latter and the radiotelephone box.

An object of the present invention is therefore to create an antenna for a portable radio device, efficiency is increased compared to antennas of this type currently known.

Another object of the present invention is to achieve an antenna of the previous type which occupies a volume at inside the weakest wearable device possible.

Finally, another object of the present invention is to make a previous type antenna in which the strand extractable as independent as possible from the housing associated radio device.

The present invention provides for this purpose an antenna of the type for portable radio device, according to claim 1.

Other features and advantages of this invention will appear in the following description of modes possible embodiments of an antenna according to the invention, these embodiments being given by way of illustration and in no way limiting.

• la figure 1 représente, en coupe partielle, une portion d'un radiotéléphone au niveau de laquelle est installée une antenne selon l'invention,
• la figure 2 est une coupe transversale du brin extractible représenté en figure 1,
• la figure 3A représente de manière schématique l'antenne hélicoïdale de la figure 1, et la figure 3B la courbe correspondante donnant l'intensité en fonction de la hauteur depuis la base de l'hélice,
• la figure 4 est un schéma équivalent de l'antenne de la figure 1 lorsque le brin extractible est en position rentrée,
• la figure 5 est un schéma équivalent de l'antenne de la figure 1 lorsque le brin extractible est en position déployée,
• la figure 6A représente de manière schématique une antenne hélicoïdale classique, la figure 6B la courbe correspondante donnant l'intensité en fonction de la hauteur depuis la base de l'hélice, et la figure 6C le schéma équivalent de cette antenne,
• la figure 7A représente de manière schématique une antenne hélicoïdale à hélice à pas variable mais à largeur constante, non conforme à la présente invention, la figure 7B la courbe correspondante donnant l'intensité en fonction de la hauteur depuis la base de l'hélice, et la figure 7C le schéma équivalent de cette antenne,
• la figure 8A représente de manière schématique une autre antenne hélicoïdale à hélice à pas variable mais à largeur constante, non conforme à la présente invention, la figure 8B la courbe correspondante donnant l'intensité en fonction de la hauteur depuis la base de l'hélice, et la figure 8C le schéma équivalent de cette antenne,
• la figure 9 représente en vue de face et en coupe partielle le brin extractible de l'antenne de la figure 1,
• la figure 10 représente en vue de face et en coupe partielle une première variante du brin extractible de l'antenne de la figure 1, dans son revêtement de protection,
• la figure 11 représente en vue de face et en coupe partielle une deuxième variante du brin extractible de l'antenne de la figure 1, dans son revêtement de protection,
• la figure 12 représente en perspective une variante possible pour le brin extractible de la figure 9,
• la figure 13A représente en vue de face une variante possible pour l'antenne hélicoïdale de la figure 1,
• la figure 13B est une vue en coupe de la paroi de l'antenne hélicoïdale de la figure 13A,
• la figure 13C représente la courbe donnant l'intensité en fonction de la hauteur depuis la base de l'hélice de l'antenne de la figure 13A,
• la figure 14A montre ce qui est obtenu à l'issue d'une étape d'une méthode possible de fabrication d'une antenne hélicoïdale telle que celle de la figure 1,
• la figure 14B montre comment procéder à l'assemblage de ce qui a été obtenu à la figure 14A.

In the following figures:

• FIG. 1 represents, in partial section, a portion of a radiotelephone at the level of which an antenna according to the invention is installed,
• FIG. 2 is a cross section of the extractable strand shown in FIG. 1,
• FIG. 3A schematically represents the helical antenna of FIG. 1, and FIG. 3B the corresponding curve giving the intensity as a function of the height from the base of the propeller,
• FIG. 4 is an equivalent diagram of the antenna of FIG. 1 when the extractable strand is in the retracted position,
• FIG. 5 is an equivalent diagram of the antenna of FIG. 1 when the extractable strand is in the deployed position,
• FIG. 6A schematically represents a conventional helical antenna, FIG. 6B the corresponding curve giving the intensity as a function of the height from the base of the propeller, and FIG. 6C the equivalent diagram of this antenna,
• FIG. 7A schematically represents a helical antenna with a variable pitch but constant width propeller, not in accordance with the present invention, FIG. 7B the corresponding curve giving the intensity as a function of the height from the base of the propeller, and FIG. 7C the equivalent diagram of this antenna,
• FIG. 8A schematically represents another helical antenna with a variable pitch propeller but of constant width, not in accordance with the present invention, FIG. 8B the corresponding curve giving the intensity as a function of the height from the base of the propeller , and FIG. 8C the equivalent diagram of this antenna,
• FIG. 9 represents in front view and in partial section the extractable strand of the antenna of FIG. 1,
• FIG. 10 represents in front view and in partial section a first variant of the extractable strand of the antenna of FIG. 1, in its protective coating,
• FIG. 11 shows in front view and in partial section a second variant of the extractable strand of the antenna of FIG. 1, in its protective coating,
• FIG. 12 shows in perspective a possible variant for the extractable strand of FIG. 9,
• FIG. 13A represents a front view of a possible variant for the helical antenna of FIG. 1,
• FIG. 13B is a sectional view of the wall of the helical antenna of FIG. 13A,
• FIG. 13C represents the curve giving the intensity as a function of the height from the base of the antenna propeller of FIG. 13A,
• FIG. 14A shows what is obtained at the end of a step of a possible method of manufacturing a helical antenna such as that of FIG. 1,
• Figure 14B shows how to assemble what was obtained in Figure 14A.

In all these figures, the common elements bear the same reference numbers.

We will first refer to Figure 1.

We see in this figure an antenna 1 according to the invention. The antenna 1 comprises a helical antenna 2 5 and an extractable strand 3.

The helical antenna 2 is partly housed in a recess 4 of a radiotelephone case 5, partially shown in Figure 1. The housing 5 consists of a insulating material, possibly metallized, and has a shape substantially parallelepiped. In order to maintain and protect the helical antenna 2, an antenna box 6 (shown in broken lines) whose base comes complete the recess 4 is used.

The helical antenna 2 is fully inserted in the antenna housing 6, and exceeds about three-quarters of its height beyond the housing 5 of the radiotelephone.

It consists more particularly of a mandrel support 7 made of an insulating material, substantially shaped cylindrical, on the outside surface of which has been deposited by a conventional metal deposition process, a propeller 8. According to the invention, the pitch of the propeller 8 is variable and decreases from its base 8B to its apex 8A. Of even, still according to the invention, the width of the track electric constituting the propeller 8 is also variable and decreases from base 8B to top 8A. We explain the reasons for such a structure and the advantages it provides hereinafter description.

The electrical length of the propeller 8 is substantially equal to half the average wavelength use.

The base 8B of the propeller 8, located at the base of the mandrel 7, is connected by means of a tongue interconnection 9 to a coaxial cable 10 supplying the helical antenna 2 located in the housing 5 of the radiotelephone and also connected to the transmitter / receiver of the latter (not shown).

There is also in the antenna housing 6 a positioning ring 11 (shown in broken lines) made of an insulating material, intended to center and maintain the helical antenna 2.

The extractable strand 3 consists of a ribbon metal 12 with a very flat C-shaped section (see Figures 2 and 9), which we will describe as flat. The length electrical tape 12 is approximately equal to half of the average wavelength of use. Ribbon 12 is in additionally inserted into a covering 13 made of an insulating material intended to protect it.

Advantageously, the extractable strand 3 comprises in addition a metal element 15 at its top 3A, this element 15 extending in a direction substantially orthogonal to the X axis of the propeller 8 (the ribbon 12 extends in a direction substantially parallel to the axis X). The element 15 is also inserted into the covering 13, and it may or may not be electrically connected to the ribbon 12. will explain later its usefulness.

The extractable strand 3 can operate in two positions. In a first position (corresponding to that illustrated in Figure 1), it is almost entirely tucked into the antenna housing 6 and into a housing adapted 14 practiced in the housing 5 of the radiotelephone. In this position, antenna 1 is of the quarter wave type (i.e. it uses box 5 as a counterweight electric), and only the helical antenna 2 is then used to the transmission and reception of radio signals. The walls of housing 14 are covered with metal 141 to constitute a shield for extractable strand 3 in retracted position.

In a second position (not shown), the strand extractable 3 is fully deployed outside the antenna box 6. In this case there is a coupling capacitive between strand 3 and the top of the antenna helical 2, so the total height of the antenna 1 and its radiation resistance are increased. In this position of the extractable strand 3, the antenna 1 is still quarter wave type.

In order to limit the travel of the extractable strand 3 during of its deployment, the lower end 13B of the covering 13 is frustoconical with its largest base diameter oriented towards the top of the antenna. The end 13B abuts against the upper wall 14A of the housing 14.

as mentioned above, a essential characteristic of the present invention resides in the fact that we use a helical antenna whose the propeller is of variable pitch, this pitch decreasing when we approaches the top of the helical antenna, that is to say as the theoretical current in a conventional helical antenna (i.e. pitch and constant width) of the same dimensions decreases. A on the one hand, this structure improves efficiency antenna 1 by ensuring better energy transfer, and on the other hand to increase the bandwidth of the antenna 1.

Indeed, this structure makes it possible to establish in the helical antenna 2 a distribution substantially trapezoidal current. This increases the resistance of radiation of the antenna, and therefore its efficiency and its bandwidth.

In the example illustrated in Figure 1, the turns of propeller 8 are in contact with each other at vertex 8A, so that we get at vertex 8A a continuous metallic surface. So the top 8A is rendered capacitive, which provides the distribution substantially trapezoidal of the current and the advantages which result. Immediately before the 8A summit, the turns of the propeller 8 constitute a tight spiraling without however to be in contact with each other. The capacity thus realized is made selfic, which increases the apparent value. In addition, achieving a capacity at the top of the helical antenna 2 facilitates and improves the capacitive coupling and adaptation between the latter and the extractable strand 3.

Thus, the variable pitch propeller makes it possible to obtain optimal adaptation and coupling conditions in two operating modes (strand retracted or deployed).

By way of example, there is shown in FIG. 6A, very schematically, a helical antenna 62 with pitch and width constant, according to the prior art. Curve 63 in Figure 6B represents the intensity of the current i as a function of the height h along the X axis of the helical antenna 62. We see that the distribution of current i is substantially triangular. Finally, Figure 6C shows the diagram equivalent of antenna 62: this antenna is equivalent to one pure inductance 64.

We see in Figure 7A, very schematically, a helical antenna 72 which could be used instead of the helical antenna 2 of FIG. 1. The turns of antenna 72 are in contact with each other at top of the latter so as to constitute a continuous metallization. Curve 73 of Figure 78, which represents the intensity of the current i as a function of the height h along the X axis, shows that the distribution of current tends towards a trapezoidal shape. Figure 7C, which represents the equivalent diagram of antenna 72, illustrates that the latter is equivalent to an inductance 74 in series with capacity 75.

Similarly, we see in Figure 8A, very schematic, a helical antenna 82 which could be used in instead of the helical antenna 2 in FIG. 1. The turns of antenna 82 are in contact with each other others at the top of the latter so as to constitute continuous metallization, and tightened without being contact with each other immediately before to reach the top. The rest of the propeller is not constant. The curve 83 of FIG. 8B, which represents the intensity of the current i as a function of the height h according to the X axis, shows that the current distribution tends to more and more (compared to Figure 7B) towards a form trapezoidal. Figure 8C, which shows the diagram equivalent of antenna 82, illustrates that the latter equivalent to a first inductance 84 (corresponding to the part of the propeller with constant pitch), in series with a second inductor 85 (corresponding to the part of tight pitch propeller) and with capacity 86 (corresponding at the top of the propeller where the turns are in contact with each other with the others).

According to an advantageous improvement of the present invention, in order to further increase the resistance of radiation from a helical antenna such as that shown in Figure 7A or Figure 8A, i.e. increase its overvoltage, we optimize the width of the electric track constituting the propeller, in order to increase the area defined by the current distribution. We obtain thus an efficiency and a bandwidth still improved for the antenna according to the invention.

The helical antenna 2 shown in Figure 1 illustrates the principles which have just been exposed. She is shown schematically in Figure 3A, and accompanied by FIG. 3B of the corresponding curve 33 representing the intensity of the current i as a function of the height according to the X axis. Note that the area between the curve 33 and the coordinate axes is further increased by relative to the surface corresponding to FIGS. 7B or 8B. This has the effect of increasing the radiation resistance and therefore the efficiency and bandwidth of the antenna.

Figures 4 and 5 show the diagrams antenna 1 equivalents respectively when the strand extractable 3 is in the retracted position and when in deployed position.

• C₁ représente la capacité cumulée ajoutée d'une part par l'élément 15 au sommet 3A du brin extractible 3 et d'autre part par la partie capacitive du sommet 8A de l'hélice 8 ; la partie de C₁ correspondant à l'élément 15 du brin 3 complète l'effet apporté par le sommet capacitif 8A de l'hélice 8,
• L_H représente l'inductance élevée due au spiralage serré immédiatement avant le sommet 8A de l'hélice 8,
• L_B représente l'inductance faible de la partie basse de l'hélice 8 ; L_B est négligeable devant L_H,
• C₂ est une capacité parasite sur la partie basse de l'hélice 8 ; elle est négligeable du fait que L_B est très petite devant L_H.

In figure 4:

• C ₁ represents the cumulative capacity added on the one hand by the element 15 to the top 3A of the extractable strand 3 and on the other hand by the capacitive part of the top 8A of the propeller 8; the part of C ₁ corresponding to the element 15 of the strand 3 completes the effect provided by the capacitive top 8A of the propeller 8,
• L _H represents the high inductance due to the tight spiraling immediately before the apex 8A of the propeller 8,
• L _B represents the low inductance of the lower part of the propeller 8; L _B is negligible compared to L _H ,
• C ₂ is a parasitic capacity on the lower part of the propeller 8; it is negligible because L _B is very small compared to L _H.

In FIG. 5, the part of C ₁ brought by the element 15 to the top 3A of the extractable strand 3 no longer has an effect when the strand 3 is in the deployed position, and the part of C ₁ brought by the top 8A propeller 8 has been taken into account in the capacitance C ₃ of coupling of the extractable strand 3 with the helical antenna 2; this coupling is high and tends to reduce the effect of L _H , which compensates for the capacity C ₄ added by the strand 3 deployed and corresponding to the antenna effect of strand 3 relative to the external environment.

Increasing the height of antenna 1 by deploying the extractable strand 3 improves, in a known manner, the efficiency of the antenna, by increasing its effective height and its radiation resistance.

Note that the extractable strand 3 is not necessarily located outside the helical antenna 2; indeed, if the support mandrel is hollow, the strand extractable can be inside the mandrel 7, this which has the advantage of saving space additional.

FIGS. 10 and 11 also show possible variants for the extractable strand 3.

More specifically, Figure 10 shows a variant usable in place of the tape 12 of the extractable strand 3 of the Figures 1, 2 and 9 (element 15 has not been shown in figure 10). Instead of using metallic tape 12, uses a metallic conductive line 1012 deposited from so as to form a crenellated line on a film in one insulating material constituting part of the covering 13. The line 1012 is embedded in the covering 13. Such a structure shortens the effective length of the strand extractable 3, while maintaining an electrical length equal to half the wavelength. This allows decrease the space occupied by the extractable strand 3 to inside of the radiotelephone case 5. As with the tape 12, one can also optionally use element 15 at the top of strand 3, to obtain the same effect as described above.

According to another possible variant for the strand extractable 3, illustrated in FIG. 11 (the element 15 has not shown in these figures, but can also be used in connection with the variant illustrated there), instead of ribbon 12 a metallic wire can be used 1112 having a crushed spring structure producing a tile effect without contact between the turns.

The spring 1112 is also embedded in a coating 13 made of an insulating material, and the advantages it provides are identical to those obtained with line 1012.

All these structures for the extractable strand 3 (ribbon 12, line 1012, spring 1112) reduce more or less the size of the extractable strand 3 in the radiotelephone case 5, which leaves more space for other essential elements of the latter. The insulating material constituting the covering 13 will be chosen at both to give strand 3 flexibility and to ensure sufficient mechanical strength for protect the metal part it contains.

According to a possible improvement for the strand extractable 3, more particularly suitable for use tape 12, the upper metal part of strand 3, located immediately before the capacitive element 15, is connected to this last by an inductive structure 16. This allows improve the efficiency in the deployed position of the strand extractable 3.

We will now give details on the manufacture of a helical antenna according to the invention.

As already indicated, the helical antenna 2, as well as all the variants which have been described, can be made by metallic deposition on a support mandrel 7. The propeller can thus be obtained according to any method classic (metallization then screen printing, metallization then masking and photolithography, according to the method described in patent application EP-0 465 658, etc.).

The propeller can be made on the external surface or internal of a mandrel made of an insulating material (when this mandrel is tubular). Preferably, if the propeller 8 is produced on the external surface of the mandrel 7 (as shown in FIG. 1), the metallization will be covered with a protective coating (not shown).

When metallizing inside of a support mandrel, the thickness of the mandrel wall preferably be low, to facilitate coupling possible capacitive with an extractable strand. Moreover, he it may be necessary to ensure the rigidity of the antenna helical thus obtained by inserting into the mandrel a any reinforcement piece of insulating material.

The proposed method of making propeller 8 by metallization is advantageous because it makes the very compact helical antenna 2, which allows this last to occupy as little space as possible inside of the radiotelephone case 5. In addition, the reproducibility of the propeller thus produced is better compared to using a wound wire.

Furthermore, the use of this method in the framework of the invention is particularly advantageous because it makes it easy to make a pitch propeller and variable width. It is well understood indeed that the realization of such a propeller using a metal wire wound, even if conceivable, is much more complex.

According to a variant proposed by the present invention in the manufacturing process of the helical antenna, we can, instead of directly metallizing on a mandrel having the required shape, make a deposit metallic on a flat and flexible insulating film 20 (see Figure 14A). The flexible film 20 can be constituted in particular from Kapton, Mylar or Duroid (registered trademarks). Her form constitutes the developed form of the final form that we want to give to the helical antenna. We eliminate then, by screen printing, photolithography or other, the parts of the metallization not necessary, so that obtain a pattern 21 such that, by joining by joining two opposite sides 20C and 20D of the film 20, a helix is obtained desired pitch and width.

Film 20 has metallized vias for this purpose 22, and on its face opposite to that comprising the pattern 21, around the metallized vias 22, metallized pellets 23 (see figure 14B) intended to ensure continuity of the whole.

The film 20 is assembled by welding on a mandrel (not shown) of desired shape (see figure 14B).

This method has the advantage of being simpler work (depositing on a flat surface is more simple to realize that depositing on a surface of revolution), and allow to give to the antenna helical any shape (frustoconical, cylindrical, of rectangular section, etc ...).

It can also be seen in FIG. 14A that the film 20 has a "tab" at its upper part 24 rectangular, with an area smaller than that of film 20, on which also shows a metallization pattern 25 comprising a full central part 26, surrounded by a spiraling 27. This tab 24 is intended to be folded down right angle when assembling film 20 on a form rectangular with rounded corners. The central part full 26 will then constitute the capacitive apex of the antenna helical, and spiraling the high inductance part.

When you only want to use a capacity of top (cf. FIG. 7A), the tab 24 can be entirely metallized by solid metallization.

The lower recess 28 of the film 20 will serve to make the interconnection tab with the coaxial cable Power.

All the variants which have just been described for the antenna according to the invention comprise a supply by coaxial cable, this coaxial cable being connected on the one hand to the helical antenna, and on the other hand the radiotelephone transmitter / receiver with which the antenna according to the invention is related.

It is possible to supply power to the antenna according to the invention in another way. So we have shown in Figures 13A and 13B a possible variant for the helical antenna 2 of FIG. 1. Here, the propeller 138 has two portions 138A and 138B. The 138A portion is consisting of a 1381 metallization, for example on the outer surface of the mandrel 7, of width and pitch variable so as to achieve a capacitive peak and a high inductance, in the same way as in figure 1. The part 138B includes a metallization 1382 on the surface exterior extending metallization 1381 but having a constant pitch and width, and metallization corresponding 1383 on the inner surface of the mandrel 7 (tubular) opposite metallization 1382 and wider than the latter.

The electrical length of the portion 138A is about a quarter of the wavelength, as is that of portion 138B.

The corresponding curve 133 giving the intensity of the current i as a function of height h along the X axis for the helical antenna 132 thus obtained is given in figure 13C.

The lower part of the helical antenna 132 serves thus both of radiating element (metallizations 1381 and 1382) and power line (1382 and 1383), the metallization 1383 corresponding to the ground conductor, that is to say to the external conductor of the coaxial supply, and the metallization 1382 corresponding to the core of the supply coaxial (when metallization 1381-1382 is located on the inner surface of the mandrel 7, the metallization 1383 is then of course at outside).

The manufacturing method of the helical antenna according to Figures 13A and 13B can be one of the methods previously described. You can also make the antenna 132 by winding, although this is much less easy.

Obviously, the invention is not limited to embodiments which have just been described.

In particular, an antenna according to the invention does not does not necessarily have an extractable strand. Indeed, such a strand is only necessary to allow the antenna to function whatever the conditions, and a such specification is not always formulated.

In addition, the arrangement adopted for the antenna according to the invention with respect to the radiotelephone housing is just one example. Other provisions are possible without departing from the scope of the present invention.

Using a metallization method to manufacturing the helical antenna according to the invention allows besides easily making circuits as constants distributed or located at the top of the antenna, or additional impedance correction elements.

It will be understood that the essential characteristic of the invention is to produce a variable pitch propeller and decreasing towards the top of the helical antenna, with a wire whose width decreases towards its top.

Claims (23)

1. An antenna (1) for portable radio devices including a helical antenna (2) coupled at its base to a transmitter/receiver, the pitch of said conductive material helix (8) constituting said helical antenna (2) being variable according to the height of the helix, and decreasing from the base (8B) of said helical antenna to its top (8A), and the electrical length of said helix (8) being substantially equal to one half-wavelength, characterized in that said helix (8) is made from a conductive material wire whose width decreases from the base of said helical antenna to its top.
2. An antenna according to claim 1 characterized in that the turns of said helix (8) at the top of said helical antenna (2) are in contact with each other to form a continuous conductive material surface so that said top (8A) is capacitive.
3. An antenna according to claim 2 characterized in that the turns of said helix immediately before said capacitive top (8A) are very close together without being in contact with each other in order to produce an inductance higher than that of the remainder of said helix (8).
4. An antenna according to either claim 2 or claim 3 characterized in that a bottom portion of said helix (8) has a constant pitch.
5. An antenna according to any of claims 1 to 4 characterized in that said helix constitutes at its top a circuit with distributed or lumped constants.
6. An antenna according to any of claims 1 to 5 characterized in that said helix (138) comprises over a part of its height a coaxial member (138B) including a central core (1382) and an outer conductor (1383), the coaxial member (138B) extending from the base of said helix (138) and having an electrical length substantially equal to one quarter-wavelength, said core (1382) extending to the top of said helix (1381) and said coaxial member (138B) being connected to the feed coaxial cable of said helical antenna (132).
7. An antenna according to any of claims 1 to 6 characterized in that it includes, in addition to said helical antenna, a half-wave retractable whip (3) mounted on said device and adapted to be capacitively coupled to said helical antenna (2) when deployed and to be decoupled from said helical antenna (2) when retracted, the lengthwise direction of said whip (3) being substantially parallel to the axis (X) of said helix (8).
8. An antenna according to claim 7 characterized in that said retractable whip (3) has a conductive material top (3A) end (15) whose length is short compared to that of said whip (3).
9. An antenna according to claim 8 characterized in that said metallic end (15) is orthogonal to the lengthwise direction of said whip (3) and is electrically connected to said whip by an inductive portion (16), the whole being inserted into an insulative material covering (13).
10. An antenna according to any of claims 7 to 11 characterized in that said whip (3) comprises a flat section conductive material strip (12) and is inserted into an insulative material covering (13).
11. An antenna according to any of claims 7 to 9 characterized in that said whip (3) is made from an insulative material flexible film (13) into which is inserted a conductive line forming a crenellated structure (1012).
12. An antenna according to any of claims 7 to 9 characterized in that said whip (3) is made from an insulative material flexible film (13) into which is inserted a conductive wire (1112) having the shape of a crushed spring.
13. An antenna according to any of claims 7 to 12 characterized in that said retractable whip (3) is inside the helix (8) forming said helical antenna (2).
14. An antenna according to any of claims 7 to 12 characterized in that said retractable whip (3) is outside the helix (8) forming said helical antenna (2).
15. An antenna according to any of claims 7 to 14 characterized in that said retractable whip (3) is entirely surrounded by a metal shield (141) when retracted into said radio device.
16. A method of manufacturing an antenna according to any of claims 1 to 15 characterized in that said helix (8) is obtained by depositing metal onto the outside surface of an insulative material former (7).
17. A method according to claim 16 characterized in that said metallic deposit is covered with a protective material.
18. A method of manufacturing an antenna according to any of claims 1 to 15 characterized in that said helix is obtained by depositing a metal onto the inside surface of a tubular insulative material former.
19. A method according to claim 18 characterized in that said former is thin.
20. A method of manufacturing an antenna according to any of claims 1 to 15 characterized in that said helix (8) is obtained by depositing metal strips (21) onto a substantially flat flexible film (20) corresponding to the developed shape of the final shape to be imparted to the helix, two opposite sides (20C, 20D) of said flexible film (20) being then welded together to obtain a helical shape of the deposit (21) and electrical continuity.
21. A method according to any of claims 16 to 20 characterized in that said former is cylindrical, frustoconical or parallelepiped shape with rounded edges.
22. A portable radio device characterized in that it includes an antenna (1) according to any of claims 1 to 15, said helical antenna (2) being disposed in the top part of a casing (5) of said device.
23. A portable radio device characterized in that it includes an antenna (1) according to any of claims 7 to 15, said helical antenna (2) being disposed in the top part of a casing (5) of said device and said retractable whip (3) being inserted in a housing (14) which is part of said casing (5) when retracted and emerging from the top part of said casing (5) when deployed.

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